Trainers are also simpler/easier to fly than combat aircraft, as well as cheaper and easier to maintain.
For example, in early WW2 training, US pilots intially flew a fixed gear/fixed pitch airplanes with low horsepower (~200) and usually docile handling characteristics. (PT17/PT19/PT22, Primary Trainers)
Then you moved on to Basic Trainers (BT-9, BT-13) that incorporated a variable pitch props and higher horsepower (~450) while retaining fixed gear. These aircraft could perform more complex maneuvers and were a bit tougher to fly.
Next came the Advanced Trainers, either AT-6 (Single Engine for the short guys headed for fighters) or maybe the AT-11 (Cessna "Bamboo Bomber") for the tall guys headed for multis. Now you got retractable gear, variable pitch and more horsepower.
From these it was on to your assignment.